Systems, methods, and apparatus for leakage current masking and ground fault detection

ABSTRACT

A leakage current masking device for use with a circuit includes at least one inductive load device coupled to the circuit and configured to supply an inductive load to the circuit, and a processor communicatively coupled to the inductive load device. The processor is configured to receive a signal representative of a current through the circuit, calculate a capacitive leakage current component of the current, and cause the inductive load device to adjust the inductive load supplied to the circuit to reduce the capacitive leakage current component.

BACKGROUND OF THE INVENTION

The embodiments described herein relate generally to circuit faultdetection and, more particularly, to detecting a ground fault in acircuit.

In a conventional electrical system, analysis of sinusoidal AC currentand voltage performance is simplified by using a phasor characterizationof the sinusoids. Such phasor characterizations generally use complexnumbers having “real” components associated with resistive elements and“imaginary” components associated with reactive elements. For example, aphasor characterization of a ground fault current in a circuit includesa reactive, imaginary current component that flows through thecapacitive elements of the electrical system, and a resistive, realcurrent component. The reactive current component is purely reactive andneither causes heating nor presents a shock hazard. Therefore, thereactive current component of the ground fault current does notnecessitate tripping of a protective device. In contrast, the resistivecomponent can cause heating and present a shock hazard. Accordingly,only the resistive of the ground fault current necessitates tripping ofthe protective device.

At least some known systems and devices for use in charging an electricdevice, such as an electric vehicle or hybrid-electric vehicle, areincapable of discriminating between capacitive-generated leakage currentand resistive ground current. Accordingly, at least some known systemsand devices are susceptible to nuisance tripping, which interruptscurrent flow to the electric device. For example, a charging system ordevice generally connects to a power distribution network through ahousehold wall power outlet, such as an outlet that is provided in agarage or carport. Most fire codes and regulations require these outletsto include a ground fault circuit interrupt (GFCI) breaker or to use aself-contained ground fault interrupt wall outlet that detects resistivecurrent. However, many electric vehicle on-board battery chargersgenerate a high leakage current that can cause a GFCI device, such as aGFCI breaker or a GFI wall outlet, to trip due to a capacitive currentand when a true resistive ground fault is not actually present. At leastsome GFCI devices can be made to nuisance trip by a capacitive currentto ground in the absence of a real, or resistive, ground faultcondition. For example, the capacitive current can exceed apredetermined current threshold of at least some known GFCI devices andresult in a nuisance trip of the GFCI device.

BRIEF DESCRIPTION OF THE INVENTION

In one aspect, a leakage current masking device is provided for use witha circuit. The leakage current masking device includes at least oneinductive load device coupled to the circuit and configured to supply aninductive load to the circuit, and a processor communicatively coupledto the inductive load device. The processor is configured to receive asignal representative of a current through the circuit, calculate acapacitive leakage current component of the current, and cause theinductive load device to adjust the inductive load supplied to thecircuit to reduce the capacitive leakage current component.

In another aspect, a charging system includes a ground fault circuitinterrupter (GFCI) configured to detect a resistive ground fault in acircuit that couples a source and a load, and a leakage current maskingdevice electrically coupled to the GFCI. The leakage current maskingdevice includes at least one inductive load device coupled to thecircuit and configured to supply an inductive load to the circuit, and aprocessor communicatively coupled to the inductive load device. Theprocessor is configured to receive a signal representative of a currentthrough the circuit, calculate a capacitive leakage current component ofthe current, and cause the inductive load device to adjust the inductiveload supplied to the circuit to reduce the capacitive leakage currentcomponent.

In another aspect, a method is provided for detecting a resistive groundfault in a circuit. The method includes receiving at a processor asignal representative of a current through the circuit, and calculatinga capacitive leakage current component and a ground fault currentcomponent of the current. The method also includes causing at least oneinductive load device to adjust an inductive load supplied to thecircuit to reduce the capacitive leakage current component, anddetecting a resistive ground fault in the circuit based on the groundfault current component.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of an exemplary charging system.

FIG. 2 is a schematic block diagram of an exemplary leakage currentmasking device that may be used with the charging system shown in FIG.1.

FIG. 3 is a flowchart illustrating an exemplary method for detecting aresistive ground fault in a circuit using the charging system shown inFIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

Exemplary embodiments of systems, methods, and apparatus for use inmasking a leakage current, such as a capacitive leakage current, and/ordetecting a ground fault are described herein. These embodimentsfacilitate discriminating between capacitive-generated leakage currentand resistive ground faults, thereby reducing nuisance tripping ofcircuit interruption devices, such as ground fault circuit interrupters.

The term “electric device” refers generally to any device that iscapable of receiving power from an electrical outlet and storing atleast a portion of that power in a battery or other electrical powerstorage device. Exemplary electric devices may include, but are notlimited to only including, electrical vehicles and electric-hybridvehicles. The above examples are exemplary only, and thus are notintended to limit in any way the definition and/or meaning of the term“electric device.”

FIG. 1 is a schematic block diagram of an exemplary charging system 100for use in charging an electric device 102. In an exemplary embodiment,electric device 102 includes a charging device 104 and one or morebatteries 106 that are electrically coupled to charging device 104. Asdescribed herein, electric device 102, charging device 104, and/orbattery 106 may be referred to as a load.

In an exemplary embodiment, charging system 100 includes a power source108, a ground fault circuit interrupter (GFCI) 110, and a leakagecurrent masking device 200. Masking device 200 is configured to beremovably coupled between GFCI 110 and charging device 104. In analternative embodiment, masking device 200 is a component of electricdevice 102. For example, masking device 200 may be a component ofcharging device 104 or may be electrically coupled to charging device104.

Moreover, in an exemplary embodiment, masking device 200 adjusts aninductive load on a circuit or a portion of a circuit, such as a lineconductor, until a capacitive leakage current flowing to ground is lessthan a predetermined threshold value or is approximately cancelled. Thethreshold value can be any desired value. For example, in someapplications the threshold value may be approximately five milliampsand, thus, the threshold value for capacitive leakage current should belower. Preferably, masking device 200 adjusts the inductive load untilthe capacitive leakage current is as close to zero as possible. Once theinductive load is added to the circuit to reduce the capacitive leakagecurrent, the circuit can be monitored for a resistive ground fault byGFCI 110. Reducing or cancelling the capacitive leakage current andmeasuring only a resistive current facilitates reducing nuisance tripsof GFCI 110.

FIG. 2 is a schematic block diagram of leakage current masking device200. In an exemplary embodiment, masking device 200 includes one or morecurrent sensors 202 and one or more voltage sensors 204. Current sensor202 measures a current, such as a residual current or a differentialcurrent, between GFCI 110 and the load. For example, current sensor 202may measure the current through a phase or line conductor 206 of acircuit 208. In an alternative embodiment, current sensor 202 measures afirst current through line conductor 206 and a second current through aneutral conductor 210 of circuit 208. The residual current can then becalculated as a difference between the first current and the secondcurrent. In another alternative embodiment, and when circuit 208includes two line conductors 206, current sensor 202 measures a firstcurrent through a first line conductor, a second current through asecond line conductor, and a third current through a neutral conductor210. The residual current can then be calculated as a sum of the firstand second currents, the second and third currents, or the first andthird currents. In an exemplary embodiment, current sensor 202 is acurrent transformer. However, any suitable current sensor may be usedthat enables measurement of the current through circuit 208. Once thecurrent is measured, current sensor 202 transmits a signalrepresentative of the measured current. In an exemplary embodiment,voltage sensor 204 measures a voltage between conductors of circuit 208.For example, voltage sensor 204 may measure a voltage across lineconductor 206 and neutral conductor 210. In an alternative embodiment,voltage sensor 204 measures a voltage across two line conductors 206.Once the voltage is measured, voltage sensor 204 generates a signalrepresentative of the measured voltage.

In an exemplary embodiment, masking device 200 also includes a signalconditioning unit 212 that is communicatively coupled to current sensor202 and/or voltage sensor 204. Signal conditioning unit 212 includeselectrical components for use in, for example and not by way oflimitation, filtering, amplifying, and/or converting the current and/orvoltage signals. In an exemplary embodiment, signal conditioning unit212 includes analog-to-digital (A/D) converters (not shown) that convertan analog signal, such as the current signal and/or the voltage signal,to a digital signal using an A/D conversion operation.

Moreover, masking device 200 includes an inductive load device 214 and aselector 216 that is operatively coupled to inductive load device 214.In an exemplary embodiment, inductive load device 214 includes aninductor with a plurality of taps 218 that enable adjustment of theinductance of the inductor. Selector 216 is a switch that selectivelycouples to a tap 218 of the inductor to adjust the amount of inductiveload on circuit 208. In an alternative embodiment in which circuit 208includes two line conductors 206 and neutral conductor, masking device200 includes multiple inductive load devices 214. In such an embodiment,each inductive load device 214 provides an inductive load to arespective line conductor 206.

Furthermore, masking device 200 includes a processor 220 that iscommunicatively coupled to signal conditioning unit 212. The term“processor” refers generally to any programmable system includingsystems and microcontrollers, reduced instruction set circuits (RISC),application specific integrated circuits (ASIC), programmable logiccircuits (PLC), and any other circuit capable of executing the functionsdescribed herein. The above examples are exemplary only, and thus arenot intended to limit in any way the definition and/or meaning of theterm “processor.” In an exemplary embodiment, processor 220 receives thecurrent signal and/or the voltage signal from current sensor 202 and/orvoltage sensor 204, respectively. Specifically, processor 220 receives adigitized current signal and/or a digitized voltage signal from signalconditioning unit 212. Processor 220 then determines a real portion ofthe current signal and an imaginary portion of the current signal. In anexemplary embodiment, the real portion of the current signal representsa resistive ground fault current component and the imaginary portionrepresents a capacitive leakage current component. It should beunderstood that, for a given circuit having a ground fault currentI_(gf), the current I_(gf) can be determined using a ratio of the realportion of the product of the residual current I_(r), which is the sumof a phase current and a neutral current, and a complex conjugate ofphase to neutral voltage V. This is expressed in Equation (1) below:

$\begin{matrix}{I_{gf} = \frac{{Re}\left\{ {\left( I_{r} \right) \times (V)} \right\}}{V}} & {{Eq}.\mspace{11mu} (1)}\end{matrix}$

Moreover, when calculating the real and imaginary portions of theresidual current, processor 220 produces a phasor representation of theline voltage by multiplying the measured line voltage by a first and asecond predetermined sinusoidal waveform. The first predeterminedsinusoidal waveform is used to generate a real portion of the linevoltage, and the second predetermined sinusoidal waveform is used togenerate an imaginary portion of the line voltage. In some embodiments,the first and second sinusoidal waveforms are selected to match afrequency of the voltage of the circuit. In an exemplary embodiment,processor 220 then produces a phasor representation of the residualcurrent by multiplying the measured or calculated residual current by athird and a fourth predetermined sinusoidal waveform. The thirdpredetermined sinusoidal waveform is used to generate the real portionof the residual current (i.e., the ground fault current), and the fourthpredetermined sinusoidal waveform is used to generate the imaginaryportion of the residual current (i.e., the capacitive leakage current).In some embodiments, the third and fourth sinusoidal waveforms areselected to match a frequency of the current of the circuit.

Based on the magnitude of the capacitive leakage current, processor 220determines whether an additional inductive load is necessary on circuit208. Processor 220 determines an amount of inductive load to be addedand causes selector 216 to adjust the inductive load on circuit 208using inductive load device 214. For example, processor 220 calculates acapacitance C using the capacitive leakage current I_(c), the linevoltage V, and a system frequency f. This calculation is illustrated byEquation (2) below:

$\begin{matrix}{C = \frac{I_{c}}{V*2\pi*f}} & {{Eq}.\mspace{11mu} (2)}\end{matrix}$

Processor 220 compares the capacitive leakage current to a thresholdvalue. If the capacitive leakage current is greater than the thresholdvalue, processor 220 transmits a command signal to selector 216 tofacilitate reducing the capacitive leakage current flowing to ground isless than the threshold value or is approximately cancelled. In responseto the signal, selector 216 controls inductive load device 214 to adjustthe inductive load on circuit 208, such as by increasing the inductiveload. In an exemplary embodiment, the inductive load is based on thecapacitance C of circuit 208 and the resonant frequency f_(r) of circuit208. The resonant frequency f_(r) of circuit 208 is calculated byprocessor 220 using Equation (3) as shown below:

$\begin{matrix}{f_{r} = \frac{1}{2\pi \sqrt{LC}}} & {{Eq}.\mspace{11mu} (3)}\end{matrix}$

After the capacitive leakage current is appropriately reduced, processor220 calculates a magnitude of the line voltage phasor, and generates acomplex conjugate of the line voltage phasor by determining a negativeof the imaginary portion of the line voltage. The residual currentphasor is then multiplied by the conjugate voltage phasor to generate aphasor representation of the ground fault current. The ground faultcurrent phasor is calculated by dividing a magnitude of the line voltagephasor to obtain the real portion of the ground fault current. TheProcessor 220 then transmits the real portion of the ground faultcurrent to GFCI 110.

GFCI 110 then determines if a true ground fault condition is present incircuit 208. For example, GFCI 110 compares the real portion of theground fault current to a threshold value or predetermined level ofground fault current. If the real portion of the ground fault current isgreater than the threshold value or predetermined level of ground faultcurrent, then a true ground fault is determined to exist. In response tosuch a determination, GFCI 110 initiates opening circuit 208 by, forexample, tripping a circuit breaker to remove the ground faultdetection.

FIG. 3 is a flowchart 300 that illustrates an exemplary method fordetecting a resistive ground fault in a circuit, such as circuit 208(shown in FIG. 2). In an exemplary embodiment, current sensor 202 (shownin FIG. 2) measures 302 a current, such as a residual current, through aconductor of circuit 208. In some embodiments, current sensor 202measures a first current through line conductor 206 (shown in FIG. 2),measures a second current through neutral conductor 210 (shown in FIG.2), and transmits first and second signals representative of the firstand second currents, respectively. In addition, voltage sensor 204(shown in FIG. 2) measures a voltage across conductors of circuit 208,such as across line conductor 206 and neutral conductor 210. Voltagesensor 204 transmits a third signal representative of the voltage. In anexemplary embodiment, signal conditioning unit 212 (shown in FIG. 2)receives the signals representative of the first current, the secondcurrent, and the voltage, and performs one or more of a filteringoperation, an amplification operation, and an A/D conversion on thesignals. Signal conditioning unit 212 then transmits 304 the signals toprocessor 220 (shown in FIG. 2).

Processor 220 receives the signals and determines whether an inductiveload on circuit 208 should be adjusted and whether a ground faultcurrent is present on circuit 208. Specifically, processor 220calculates 306 a capacitive leakage current component of the residualcurrent and a ground fault current component of the residual current.For example, as described in greater detail above, processor 220calculates an imaginary portion of the residual current, whichrepresents the capacitive leakage current component. Moreover, processor220 calculates a real portion of the residual current, which representsthe ground fault current component. Processor 220 compares 308 thecapacitive leakage current component to a first threshold value. Basedon the comparison, processor 220 determines 310 whether the capacitiveleakage current component is greater than the first threshold value. Ifthe capacitive leakage current component is not greater than the firstthreshold value, processor 220 waits for a next residual current signalto be transmit by signal conditioning unit 212. When the capacitiveleakage current component is greater than the threshold value, processor220 calculates 312 an amount of inductive load to be added to circuit208 to approximately cancel the capacitive leakage current component.Processor 220 transmits a command signal to selector 216 (shown in FIG.2) that includes an amount of inductive load to be added to circuit 208to approximately cancel the capacitive leakage current component. Inresponse to the command signal, selector 216 causes inductive loaddevice 214 (shown in FIG. 2) to adjust the inductive load.

When the capacitive leakage current component has been approximatelycancelled or is at least less than the first threshold value, processor220 transmits a signal representative of the ground fault currentcomponent to GFCI 110 (shown in FIG. 2). GFCI 110 compares 314 theground fault current component to a second threshold to determine 316whether a ground fault is present on circuit 208. If a ground fault ispresent, GFCI 110 interrupts 318 current flow through circuit 208.

Exemplary embodiments of systems, methods, and apparatus for detecting aground fault and/or masking a leakage current are described above indetail. The systems, methods, and apparatus are not limited to thespecific embodiments described herein but, rather, operations of themethods and/or components of the systems and/or apparatus may beutilized independently and separately from other operations and/orcomponents described herein. Further, the described operations and/orcomponents may also be defined in, or used in combination with, othersystems, methods, and/or apparatus, and are not limited to practice withonly the systems, methods, and apparatus as described herein.

Although the present invention is described in connection with anexemplary power distribution system environment, embodiments of theinvention are operational with numerous other general purpose or specialpurpose power distribution system environments or configurations. Thepower distribution system environment is not intended to suggest anylimitation as to the scope of use or functionality of any aspect of theinvention. Moreover, the power distribution system environment shouldnot be interpreted as having any dependency or requirement relating toany one or combination of components illustrated in an exemplaryoperating environment.

The order of execution or performance of the operations in theembodiments of the invention illustrated and described herein is notessential, unless otherwise specified. That is, the operations may beperformed in any order, unless otherwise specified, and embodiments ofthe invention may include additional or fewer operations than thosedisclosed herein. For example, it is contemplated that executing orperforming a particular operation before, contemporaneously with, orafter another operation is within the scope of aspects of the invention.

When introducing elements of aspects of the invention or embodimentsthereof, the articles “a,” “an,” “the,” and “said” are intended to meanthat there are one or more of the elements. The terms “comprising,”including,” and “having” are intended to be inclusive and mean thatthere may be additional elements other than the listed elements.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they have structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements with insubstantial differencesfrom the literal language of the claims.

1. A leakage current masking device for use with a circuit, said leakagecurrent masking device comprising: at least one inductive load devicecoupled to the circuit and configured to provide an inductive load tothe circuit; and a processor communicatively coupled to said at leastone inductive load device, said processor configured to: receive asignal representative of a current through the circuit; calculate acapacitive leakage current component of the current; and cause said atleast one inductive load device to adjust the inductive load provided tothe circuit to reduce the capacitive leakage current component.
 2. Aleakage current masking device in accordance with claim 1, furthercomprising a current sensor communicatively coupled to said processor,said current sensor configured to measure the current through thecircuit and transmit the signal representative of the current to saidprocessor.
 3. A leakage current masking device in accordance with claim2, wherein the circuit includes at least one line conductor and aneutral conductor, said current sensor is further configured to measurea first current through the at least one line conductor, measure asecond current through the neutral conductor, transmit a first signalrepresentative of the first current to said processor, and transmit asecond signal representative of the second current to said processor. 4.A leakage current masking device in accordance with claim 3, whereinsaid processor is configured to calculate the current through thecircuit as a sum of the first current and the second current.
 5. Aleakage current masking device in accordance with claim 2, furthercomprising a signal processing unit communicatively coupled to saidcurrent sensor and said processor, said signal processing unit isconfigured to receive the signal representative of the current throughthe circuit from said current sensor and to apply at least one of afiltering operation and an analog-to-digital conversion operation to thesignal.
 6. A leakage current masking device in accordance with claim 1,wherein said processor is further configured to compare the capacitiveleakage current component to a threshold value.
 7. A leakage currentmasking device in accordance with claim 6, wherein said processor isfurther configured to cause said at least one inductive load device toincrease the inductive load on the circuit when the capacitive leakagecurrent component is greater than the threshold value.
 8. A leakagecurrent masking device in accordance with claim 1, further comprising aselector communicatively coupled to said processor and said at least oneinductive load device, said processor is further configured to transmita command signal to said selector, wherein said selector is configuredto cause said at least one inductive load device to adjust the inductiveload supplied to the circuit in response to the command signal.
 9. Acharging system comprising: a ground fault circuit interrupter (GFCI)configured to detect a resistive ground fault in a circuit that couplesa source to a load; and a leakage current masking device electricallycoupled to said GFCI, said leakage current masking device comprising: atleast one inductive load device coupled to the circuit and configured tosupply an inductive load to the circuit; and a processor communicativelycoupled to said at least one inductive load device, said processorconfigured to: receive a signal representative of a current through thecircuit; calculate a capacitive leakage current component of thecurrent; and cause said at least one inductive load device to adjust theinductive load supplied to the circuit to reduce the capacitive leakagecurrent component.
 10. A charging system in accordance with claim 9,wherein said leakage current masking device further comprises a currentsensor communicatively coupled to said processor, said current sensor isconfigured to measure the current through the circuit and transmit thesignal representative of the current to said processor.
 11. A chargingsystem in accordance with claim 10, wherein the circuit includes atleast one line conductor and a neutral conductor, said current sensor isfurther configured to measure a first current through the at least oneline conductor, measure a second current through the neutral conductor,transmit a first signal representative of the first current to saidprocessor, and transmit a second signal representative of the secondcurrent to said processor.
 12. A charging system in accordance withclaim 11, wherein the current through the circuit includes a residualcurrent, said processor is configured to calculate the residual currentthrough the circuit by summing the first current and the second current.13. A charging system in accordance with claim 10, wherein said leakagecurrent masking device further comprises a signal processing unitcommunicatively coupled to said current sensor and said processor, saidsignal processing unit is configured to receive the signalrepresentative of the current through the circuit from said currentsensor and to apply at least one of a filtering operation and ananalog-to-digital conversion operation to the signal.
 14. A chargingsystem in accordance with claim 9, wherein said processor is furtherconfigured to compare the capacitive leakage current component to athreshold value.
 15. A charging system in accordance with claim 14,wherein said processor is further configured to cause said at least oneinductive load device to increase the inductive load on the circuit whenthe capacitive leakage current component is greater than the thresholdvalue.
 16. A charging system in accordance with claim 9, wherein saidleakage current masking device further comprises a selectorcommunicatively coupled to said processor and said at least oneinductive load device, said processor is further configured to transmita command signal to said selector, and said selector configured to causesaid at least one inductive load device to adjust the inductive loadsupplied to the circuit in response to the command signal.
 17. A methodfor detecting a resistive ground fault in a circuit, said methodcomprising: receiving at a processor a signal representative of acurrent through the circuit; calculating, by the processor, a capacitiveleakage current component and a ground fault current component of thecurrent; causing, by the processor, at least one inductive load deviceto adjust an inductive load supplied to the circuit to reduce thecapacitive leakage current component; and detecting a resistive groundfault in the circuit based on the ground fault current component.
 18. Amethod in accordance with claim 17, further comprising applying at leastone of a filtering operation and an analog-to-digital conversionoperation to the signal representative of the current.
 19. A method inaccordance with claim 17, wherein causing at least one inductive loaddevice to adjust an inductive load comprises comparing the capacitiveleakage current component to a first threshold value and causing the atleast one inductive load device to increase the inductive load on thecircuit when the capacitive leakage current component is greater than orequal to the first threshold value.
 20. A method in accordance withclaim 19, wherein detecting a resistive ground fault comprises comparingthe ground fault current component to a second threshold value.